1. Field of the Invention
The invention relates to a control device and a method for controlling an automatic transmission, which prevent causing discomfort to an occupant by a shock as a result of torsion energy that has been accumulated in a drive transmission system involved in a sharp braking, used specifically in a rear-wheel drive vehicle (FR type, that is, Front engine Rear drive type) that has a propeller shaft.
2. Description of the Related Art
An automatic transmission that is equipped with an electromagnetic valve and that receives an external electric signals from the components around and adjusts variables regarding gearshift operations, for example, gear step, oil pressure level, constant value and timing during shift operations, etc., through the use of an ECU (Electronic Control Unit), has been put into practical use. Due to the ECU, it is possible to reliably and promptly shift operational states of the automatic transmission to various states. Furthermore, because the ECU incorporates a CPU (Central Processing Unit), the automatic transmission may be controlled by programs. Therefore, by changing the programs or various constants, operational states of the automatic transmission may be precisely set. Hence, an ECU enables an automatic transmission to perform in optimum manners by corresponding to operational states of a vehicle and engine loads. It is to be noted herein that the operational states of a vehicle include vehicle speed, steering operation, frequency and level of acceleration/deceleration, state of road surface, etc. Furthermore, the engine loads include rotation speed of the engine, throttle opening degree, accelerator pedal depression amount, torques of the input and output shafts of the engine and an automatic transmission, etc.
Still further, levels of oil pressure supplied to friction engagement elements (clutches and brakes) provided within an automatic transmission are precisely adjusted in conformance with the operational states of a vehicle and the engine loads. Thus, the automatic transmission is able to restrain shock and reduce wear on the friction engagement elements, and therefore accomplishing shifting to be prompt and smoothened.
An automatic transmission constitutes a gear shift mechanism, that is linked to an engine via a torque converter or the like involves a plurality of power transmission paths. For example, an automatic transmission is constructed so as to automatically switch a power transmission path, that automatically switches the gear ratio, based on accelerator operation amount and a vehicle speed. A vehicle consists an automatic transmission is provided with a shift lever that sets the shift positions, for example, a reverse position, a neutral position, a progressing position, etc. by a driver. Of the shift positions set, the progressing position is usually selected to perform an automatic gear shift control.
In a vehicle consists an automatic transmission, while the vehicle is at a stop with a progressing position of the automatic transmission is set, a so called creep phenomenon occurs due to drive force from an engine in an idling state is transmitted via a torque converter to the transmission, and then is transmitted to wheels. A creep phenomenon, for example, allows a vehicle to smoothly start from a stop on an uphill road, and the like, and is therefore very useful under a certain condition. However, when the vehicle is kept at a stop, a creep phenomenon is unnecessary, and the creep force due to a creep phenomenon is suppressed by operating brakes of the vehicle. Thus, because the creep force due to a creep phenomenon caused by drive force from an engine is suppressed by brakes, the fuel consumption of an engine correspondingly declines, which is a problem of the aforementioned vehicle.
To overcome this problem, Japanese Patent Application Publication No. JP-A-2000-310318, Japanese Patent Application Publication No. JP-A-2000-310319, and Japanese Patent Application Publication No. JP-A-11-193866 disclose a neutral control in which while a vehicle is at a stop with a progressing position of an automatic transmission is selected, a brake pedal depressed and therefore the brakes engage, and an accelerator pedal being in a substantially non-operated state, a progressing clutch is released during the progressing position of the automatic transmission is kept, so that friction engagement elements are in a nearly neutral state, thereby attempting to improve the fuel consumption.
In an FR vehicle having a propeller shaft as a mechanical element in the drive transmission system, the drive force from the drive wheels (rear wheels) is added and therefore torsion energy is more likely to be accumulated in a drive transmission system.
During the vehicle is braking, the drive wheels drive the engine. The torsion energy that occurs is residual energy obtained by subtracting the amount of braking energy from the entire kinetic energy. This residual energy causes flexure in the drive transmission system. Since the drive transmission system and the engine are mechanically connected, the residual energy is accumulated as flexure in elastic members, such as rubber pieces and the like in the engine mount system, or the like. During braking of the vehicle, the engine mount system flexes, the power train (including the engine and the transmission) tends to move forward direction of the vehicle. Due to this, the coupling of the transmission and the propeller shaft become loose. However, the aforementioned coupling can instead become a pushed-in state depending on the mount supporting method and the construction of the power train or the drive train.
If the neutral control is executed when the coupling therebetween being loose as described above, the residual energy is released and emitted, so that the power train that has moved forward returns to the regular position. However, the coupling not always smoothly slides from the state of loose to the regular state of the coupling between the transmission and the propeller shaft. Conversely, if the splines of the transmission and the splines of the propeller shaft are meshed with each other, and are momentarily or partially anchor to each other, the residual energy is released all at once, instead of being gradually released as the splines of the transmission and the splines of the propeller shaft slide. As a result, a noise or a shock occurs to the vehicle.
As in the neutral control, normally an input clutch is released while the vehicle is at a complete stop with a brake pedal depressed. If there is residual energy accumulated in a drive transmission system during a braking operation preceding the neutral control, a shock occurs during the state of complete stop of the vehicle, irrespectively of a driver's operation, since the neutral control is started irrespectively of the driver's operation. In particular, the occurrence of a shock to the vehicle during a state where a driver has not done any operation is not preferable.
However, this problem is not mentioned in any of the foregoing related-arts. Furthermore, the attempt to solve the problem by providing a flexible coupling as an interference member in the drive transmission system or by treating the surface of the sliding portions of the coupling portions to dramatically reduce friction coefficient result in increased cost. If the neutral control of the transmission at the time of stop of the vehicle is prohibited, the problem of the shock may be avoided, but the effect of improving fuel consumption cannot be attained.